PROJECT SUMMARY/ABSTRACT Homeostatic signaling systems are believed to interface with the mechanisms of learning-related plasticity to achieve stable, yet flexible, neural function and animal behavior. As such, homeostatic signaling systems are also widely believed to participate in the cause and progression of neurological disease. Yet, direct connections to disease can only be achieved once there is a clear understanding of the cellular and molecular mechanisms that drive homeostatic plasticity. The homeostatic modulation of presynaptic neurotransmitter release is an evolutionarily conserved form of homeostatic signaling that is observed in organisms ranging from fly to human. This form of homeostatic plasticity is under bi-directional control. Homeostatic signaling mechanisms can potentiate neurotransmitter release or depress neurotransmitter release, thereby stabilizing synaptic transmission and neural function in response to a wide range of perturbations. In this grant we identify the very first molecular mechanisms responsible for presynaptic homeostatic depression in any organism. In a large-scale, electrophysiology-based, unbiased, forward genetic screen we have identified a secreted, Ig-domain containing protein that is necessary for presynaptic homeostatic depression. Furthermore, two interacting extracellular proteins have also been identified. Thus, we may have identified a novel, trans- synaptic signaling system for the control of homeostatic depression. In this grant, we have also identified a separate molecular mechanism that limits the extent of homeostatic depression. We have identified a synaptic kinase and auxiliary subunit of the presynaptic calcium channel that are necessary to prevent over- compensation. In the absence of these genes, homeostatic depression proceeds unchecked causing a dramatic impairment of neurotransmission. To date, nothing is known about how homeostatic signaling is limited in any system. Thus, we may have identified a novel form of homeostatic feedback control that limits the extent of presynaptic homeostatic depression. In conclusion, we propose to characterize completely novel mechanisms of synaptic modulation with potential relevance to the cause and progression of neurological disease.